The invention relates to an apparatus for injecting fluids.
Apparatuses for injecting fluids in the form of infusion pumps are constructed as flexible tube pumps or as pumps without tube with separate delivery chambers. In each case the continuously deliverable fluid volume is limited by the method technology. Limitation results in hose or flexible tube pumps also from the length of the delivery path and in pumps with separate delivery chamber from the volume thereof. Consequently, along with or after the delivery of the limited volume replenishment of said volume is necessary.
If a delivery of a fluid is to take place with constant flow rate a constant pressure must be maintained in the patient-side part of the transfusion apparatus.
The replenished volume is under hydrostatic pressure which is governed by the vertical level difference between fluid reservoir and delivery element. When an infusion pump is used for its intended purpose this hydrostatic pressure is always less than the fluid pressure in the delivery direction, i.e. in the patient-side part of the transfusion apparatus.
An apparatus for injecting liquids into the human body which uses a flexible conduit delivery system is known for example from EP-A-0 024 431. In this apparatus a specifically formed pump chamber is employed which is constructed as one-way pump chamber and has two so-called flexible roll diaphragm pump chambers which in the unloaded state define a predetermined volume which is reduced during the pumping operation by correspondingly formed pump rams. To achieve the desired mode of operation, such an infusion pump has two pump rams as well as an inlet valve and an outlet valve. Furthermore, a third valve piston is necessary which is not driven but moves in a pressure acquiring chamber of the pump chamber in dependence upon a fluid pressure.
If it is assumed that the delivery with constant flow rate is fulfilled via the delivery volume, finite due to the method technology, with a typical flexible delivery conduit infusion pump problems are still encountered on changing to the next delivery volume. This critical instant is reached whenever the inlet valve of a typical flexible delivery conduit infusion pump must be closed to enable the outlet valve to be opened. In a typical flexible delivery conduit infusion pump the change from one delivery phase to the next delivery phase therefore involves a temporary pressure drop in the patient-side part of the transfusion apparatus and this results in a reduction of the flow rate, i.e. a reduction of the delivery volume per unit time. This in turn reduces the accuracy of the delivery rate in particular at small values and with small measuring times of the delivery rate involves high deviations. This means in other words that small delivery rates must be maintained over a long period of time in order to achieve the desired volume delivered in unit time with an acceptable accuracy at all.
A further pump, which is identical to that of EP-A-24431, is described in GB patent 2,065,789. In this respect, similar problems arise in this case as well.
It is therefore the problem of the present invention to provide an apparatus for injecting fluids into the human body which uses a flexible conduit delivery system and which also permits an extremely uniform delivery of the medium with simultaneously small apparatus expenditure.